A CMOS active pixel is a semiconductor device capable of converting optical images into electronic signals. Such active pixels may be used in imaging devices such as video cameras. The CMOS active pixel comprises a photo site and associated read-out and resetting electronics. As photons strike a photo site, free charge carriers are generated in an amount linearly proportional to the incident photon radiation. These photogenerated charge carriers are collected in the photo site via a photo gate. The collected charge carriers form an electrical charge packet. The amount of charge contained in the charge packet is thus a representation of the intensity of the light striking the photo site.
The charge packet is stored in the photo site in spatially defined depletion regions or potential wells in the semiconductor substrate beneath the photo site. The charge packet must be moved from the depletion region under the photo site to the output circuitry for processing. This move may be accomplished by transferring the charge packet to a second, adjacent well that is located beneath a transfer gate. The transfer gate well receives the charge from the photo gate well and further transfers it to the output circuitry for processing.
Charge packet transfer between wells typically occurs in the following manner. The near-surface potential within the semiconductor can be controlled by the potential of an electrode that is in close proximity to the semiconductor surface. If closely spaced electrodes are at different voltages, they will form potential wells of different depths. Free positive charges, for example, holes, will move from the region of higher potential to the one of lower potential. Free negative charges, for example, electrons, will move from the region of lower potential to one of higher potential. Accordingly, the potential on the photo gate and transfer gate may be adjusted to effect charge movement.
There is necessarily a gap between the photo gate and the transfer gate. If the gap is not narrow enough, the surface potential under the gap will constitute an electrical potential barrier for the transfer of the charge packet. If this occurs, there is no charge transfer. In other words, there is no signal read-out. Photolithographic techniques presently dictate that a two layer polysilicon structure is required to achieve a sufficiently narrow gap between surface electrodes.
It would be desirable to form an active pixel using a process wherein only one polysilicon deposition is required. This simple process would result in lower per chip fabrication cost. Furthermore, it would be desirable to operate such a pixel in a manner that minimizes hardware requirements while maintaining optimal pixel performance.